In December 1952, the Great Smog that settled over the city of London for four deadly days claimed the lives of more than 4,000 people that winter. In 1948, a similar soot-filled cloud was responsible for dozens of deaths in Donora, Pennsylvania.

Particle pollution - especially fine particles - contains microscopic solids or liquid droplets that are so small that they can get deep into the lungs and cause serious health problems. Numerous scientific studies have linked particle pollution exposure to a variety of problems, including [aggravated asthma, chronic bronchitis, irregular heartbeat and premature death in people with heart or lung disease] . . . People with heart or lung diseases, children and older adults are the most likely to be affected by particle pollution exposure. However, even if you are healthy, you may experience temporary symptoms from exposure to elevated levels of particle pollution.

Today, the United States has vastly improved the air quality in most cities and surrounding areas, courtesy of the Clean Air Act and equally important state and local efforts to prevent people from pumping pollutants into the atmosphere.

In major U.S. cities, concentrations of carbon monoxide and particulate matter have fallen by more than 80% since 1970, according to the EPA.

The result: the average American's life-expectancy increased about five months between 1980 to 2000 as a result of decreased air pollution, according to a 2009 study in the New England Journal of Medicine.

And yet not all of the world's urban air is clean. On the contrary, many of the world's city dwellers are still choking on soot-filled skies.

The World Health Organization's Urban Outdoor Air Pollution database contains results of pollution monitoring from almost 1,100 cities in 91 countries for the period from 2003 to 2010, with the majority of values for the years 2008 and 2009. The following list ranks the 17 cities with the highest annual mean concentration of fine particulate matter – particles smaller than 2.5 microns.

There are two important caveats to keep in mind when reviewing this list. First, some cities do not collect information or report on its outdoor air quality, which means a few of the world's worst air-pollution offenders are likely missing from this list. Second, as the WHO emphasizes, "cities that collect and disseminate information on outdoor air quality need to be praised for their action, [which] is the first crucial step to identify if there is an outdoor air pollution problem and to begin to take corrective action."